Rapid dielectric permittivity measurements at microwave frequencies

The measurement of the real and imaginary components of the dielectric permittivity of a material at microwave frequencies usually involves the determination of phase shift and amplitude change in the Roberts-von Hippel technique¹, or the perturbation of resonance in a cavity.² Disadvantages of the former technique include the complicated mathematics necessary to solve the equations of the system, while the latter technique has only limited application for low-loss materials and requires a careful match between sample shape and cavity. A third method utilizes a traveling-wave probe in the specimen. Although this eliminates most of the mathematics of the Roberts-von Hippel method, it is seldom used because it applies to liquids only. Its accuracy suffers from the necessity of mechanical position measurements, made more difficult by the simultaneous need for seals as well as perfect alignment of the probe. A variation of the technique³ uses a fixed position probe and a variable-length liquid cell.